1. Field of the Invention
The present invention relates to a high-frequency input/output terminal for a package for housing a high-frequency semiconductor element for the millimeter wave region or the like, and also to a package for housing a high-frequency semiconductor element and using the high-frequency input/output terminal.
2. Description of the Related Art
A package for housing a high-frequency semiconductor element houses a high-frequency semiconductor element using a high-frequency signal in the micrometer wave region, the millimeter wave region, or the like, in a hermetically sealed manner. A signal input/output portion of such a package employs a terminal structure to which a transmission line such as a microstrip line or a strip line is joined and which is hermetically sealed while a semiconductor element is housed in the package.
An example of the configuration of such an input/output terminal is shown in FIGS. 19A, 19B, and FIG. 19A is a plan view and FIG. 19B is a sectional view taken along line I--I of FIG. 19A, in which lower dielectric substrate 1 is made of ceramics or the like. An upper dielectric substrate 2 is made of ceramics or the like and joined to the upper face of the lower dielectric substrate 1, and serves as a part of a case wall of a package. A line conductor 3 is formed on the upper face of the lower dielectric substrate 1. Side face ground layers 4 are formed on the side faces of the lower and upper dielectric substrates 1 and 2. A bottom face ground layer 5 is formed on the bottom face of the lower dielectric substrate 1. An upper-face ground layer 6 is formed on the upper face of the upper dielectric substrate 2. The prior art of FIG. 7 is the configuration of an input/output terminal used for so-called metal wall type packages.
According to this input/output terminal, the matching of the characteristic impedance of the line conductor 3 is attained by changing the line width of the portion sandwiched between the lower dielectric substrate 1 and the upper dielectric substrate 2 and corresponding to the strip line, with respect to that of the portions in front and in rear of the portion and corresponding to a microstrip line, thereby realizing low return and insertion losses. When an input/output terminal having the bottom face ground layer 5 and the side face ground layers 4 on the sides of the dielectric substrates 1 and 2 is embedded in a cutaway part formed in a metal substrate of the package, the isolation characteristics between the line conductor 3 and a line conductor of another input/output terminal which is juxtaposed to the line conductor 3 are improved.
In another input/output terminal, a ground pattern is disposed on both sides of a line conductor as shown in FIGS. 8A and 8B. FIG. 8A is a plan view, and FIG. 8B is a section view taken along the line B--B. Referring to these figures, a lower dielectric substrate 7, an upper dielectric substrate 8, a line conductor 9, a bottom face ground layer 10, and an upper-face ground layer 11 are configured in the same manner as the lower dielectric substrate 1, the upper dielectric substrate 2, the line conductor 3, the bottom face ground layer 5, and the upper-face ground layer 6 of FIG. 7, respectively. Ground patterns 12 are formed on the lower dielectric substrate 7 so as to laterally sandwich the line conductor 9. Through conductors 13 are through hole conductors or the like through which the ground patterns 12 are connected to the bottom face ground layer 10. Through conductors 14 are through holes or the like through which the ground patterns 12 are connected to the upper-face ground layer 11. The prior art of FIG. 8 has the configuration of an input/output terminal used for the so-called ceramic wall type packages. According to this input/output terminal, in the same manner as the input/output terminal shown in FIG. 7, the matching of the characteristic impedance of the line conductor 9 is attained so as to realize low return and insertion losses. Furthermore, the isolation characteristics are improved by surrounding the line conductor 9 with the ground patterns 12, the through conductors 13 and 14, the bottom face ground layer 10, and the upper-face ground layer 11.
In each of the high-frequency input/output terminals, the lower dielectric substrate 1 or 7 and the upper dielectric substrate 2 or 8 which constitute the strip line portion are usually made of the same dielectric material and configured as dielectric members having a substantially same thickness.
According to such a high-frequency input/output terminal of the prior art, in the region (the microwave region) wherein the frequency is relatively low among high frequencies, the transmission characteristics for a high-frequency signal are excellent because the characteristic impedance of the microstrip line portion is matched with that of the strip line portion.
However, for example, in a higher frequency region wherein the frequency is higher than 30 GHz (the millimeter wave region) there arises the following problem. In order to match the characteristic impedance of the line conductor 3, 9 in the strip line portion with that in the microstrip line portion and further suppress a higher mode, it is necessary to decrease the thickness of the lower dielectric substrate 1, 7 and additionally, since a width d1 of the line conductor in the strip line portion is very small and an unstable state is brought, it is necessary to set a length d2 of the line conductor 3, 9 in the strip line portion to 1/2n (n is a natural number) of the wavelength of a high-frequency signal to be transmitted via the input/output terminal, with the result that the length d2 of the line conductor 3, 9 in the strip line portion becomes very short. Consequently, the strength of the input/output terminal part is extremely lowered. Even when the length is designed to be 1/2n of the wavelength, the transmission mode in the strip line portion is substantially different from that in the microstrip line portions in front and in rear of the strip line portion because the input/output terminal part has a complex three-dimensional shape and the shape is dispersedly produced. This produces a further problem in that the return and insertion losses are increased and the transmission characteristics for a high-frequency signal are impaired.
The configuration shown in FIG. 8 has further problems in that it is difficult to produce the terminal because the small through conductors 13, 14 must be formed in the dielectric substrates 7 and 8, and that, because of the shield due to the through conductors, the return and insertion losses are larger as compared with the case of a planar shield.